1. Field of the Invention
The present invention relates to a surface emitting semiconductor laser which emits laser light in a direction perpendicular to a semiconductor substrate, and a method of manufacturing the same.
2. Description of Related Art
A surface emitting semiconductor laser is a semiconductor laser which emits laser light in a direction perpendicular to a semiconductor substrate, with a resonator provided on a semiconductor substrate in a direction perpendicular to the semiconductor substrate. This resonator generates then emits the laser light, and comprises a reflecting layer, active layer, and reflecting layer, in that order.
As one way of increasing the efficiency of a surface emitting semiconductor laser, it has been proposed to form a current constriction layer in a part of the reflecting layer, and this construction is widely adopted. This current constriction layer is formed by first forming a current constriction layer formation layer (a layer for forming the current constriction layer) from an AlAs layer on part of the reflecting layer, then oxidizing the current constriction layer formation layer from the lateral surface. Here the within the current constriction layer an unoxidized part of the current constriction layer formation layer remains. As a result, the current constriction layer has an inner edge and an outer edge, in a so-called doughnut shape.
Outstanding characteristics that can be cited of a surface emitting semiconductor laser compared with an edge-emitting laser, are that the direction of laser emission is uniform, and can be made small. Therefore, a surface emitting semiconductor laser holds great promise for application as a light source in optic fiber communications or optical parallel data processing.
In particular, when for example a surface emitting semiconductor laser is used as the light source in optical communications using optic fiber, when the coupling efficiency with the optic fiber is taken into consideration, the laser light emitted from a surface emitting semiconductor laser preferably has a single lateral mode, and a beam shaped close to a true circle. To obtain a surface emitting semiconductor laser with a beam of such a shape, it is preferable for the inner edge of the current constriction layer described above to be more close to a true circle.
However, as described above, since the current constriction layer is formed by oxidation of the current constriction layer formation layer from the lateral surface, because of the oxidation conditions it is difficult to control the shape of the inner edge to be closer to a true circle.
The objective of the present invention is the provision of a surface emitting semiconductor laser having a single transverse mode, and such that laser light with a beam shape closer to a true circle can efficiently be obtained, and a method of manufacturing thereof.
(A) According to the present invention, there is provided a surface emitting semiconductor laser which has a semiconductor substrate having Miller indices (100), and a resonator formed on the semiconductor substrate, to emit laser light from the resonator in a direction perpendicular to the semiconductor substrate, wherein:
a pillar-form semiconductor deposition is provided in at least a part of the resonator;
the pillar-form semiconductor deposition includes a current constriction layer;
a periphery of the pillar-form semiconductor deposition has arc-shaped portions which are similar to parts of an inner edge of the current constriction layer, in a plane including the current constriction layer; and
the arc-shaped portions are located in the  less than 010 greater than  direction,  less than 001 greater than  direction,  less than 0-10 greater than  direction and  less than 00-1 greater than  direction from the center of the inner edge of the current constriction layer.
In the present invention, xe2x80x9csemiconductor substrate having Miller indices (100)xe2x80x9d refers not only to a semiconductor substrate having Miller indices (100), but also to a substrate having a substrate surface inclined at a small angle (up to about 5 degrees) to the (100) plane.
In addition, xe2x80x9c less than 010 greater than  direction,xe2x80x9d xe2x80x9c less than 001 greater than  direction,xe2x80x9d xe2x80x9c less than 0-10 greater than  direction,xe2x80x9d and xe2x80x9c less than 00-1 greater than  directionxe2x80x9d refer to the crystalline orientation in a semiconductor substrate having Miller indices (100).
By means of this surface emitting semiconductor laser, laser light with a beam shape closer to a true circle can be obtained. This is described in more detail in the embodiments of the present invention.
The surface emitting semiconductor laser has some features such as following examples (1) to (9).
(1) The arc-shaped portions and the parts of the inner edge of the current constriction layer may be formed in approximately concentric circles.
(2) The arc-shaped portions may be formed by splitting the inner edge of the current constriction layer in the  less than 01-1 greater than  direction,  less than 011 greater than  direction,  less than 0-11 greater than  direction and  less than 0-1-1 greater than  direction from the center of the inner edge, and by projecting the split parts of the inner edge of the current constriction layer respectively in the  less than 010 greater than  direction,  less than 001 greater than  direction,  less than 0-10 greater than  direction and  less than 00-1 greater than  direction.
In the present invention, xe2x80x9c less than 01-1 greater than  direction,xe2x80x9d xe2x80x9c less than 011 greater than  direction,xe2x80x9d xe2x80x9c less than 0-11 greater than  direction,xe2x80x9d and xe2x80x9c less than 0-1-1 greater than  directionxe2x80x9d refer to the crystalline orientation in a semiconductor substrate having Miller indices (100).
By means of this construction, the above described benefit can be obtained.
(3) The distances from the center of the inner edge of the current constriction layer to the arc-shaped portions may be approximately equal.
(4) The arc-shaped portions may have the shape of the inner edge of the current constriction layer divided into fourth.
(5) The periphery of the pillar-form semiconductor deposition except for the arc-shaped portions may be formed outside line segments joining adjacent of the arc-shaped portions, in the plane including the current constriction layer. By means of this construction, when the pillar-form semiconductor deposition is oxidized from the lateral surface to form the current constriction layer, the shape of the current constriction layer can be controlled to be a circle.
(6) The periphery of the pillar-form semiconductor deposition except for the arc-shaped portions may comprise curved lines, in the plane including the current constriction layer.
(7) The periphery of the pillar-form semiconductor deposition except for the arc-shaped portions may comprise straight lines, in the plane including the current constriction layer.
(8) A strain application portion which generates a strain in the active layer may be formed in the pillar-form semiconductor deposition; and
the strain application portion may be formed on the periphery of the pillar-form semiconductor deposition except for the arc-shaped portions, in the plane including the current constriction layer.
By means of this construction, by forming the strain application portion in the pillar-form semiconductor deposition, the polarization of the laser light can be controlled.
(9) The inner edge of the current constriction layer may be a true circle.
(B) According to the present invention, there is provided a method of manufacturing a surface emitting semiconductor laser comprising the steps of:
(a) laminating a semiconductor deposition layer to form a layer for forming a current constriction layer on a surface of a semiconductor substrate having Miller indices (100);
(b) etching the semiconductor deposition layer to form a pillar-form semiconductor deposition, in which:
in the plane including the layer for forming the current constriction layer, an inner edge of the current constriction layer is designed to have a predetermined shape; and a periphery of the pillar-form semiconductor deposition includes arc-shaped portions which are similar to parts of the designed inner edge of the current constriction layer and located in the  less than 010 greater than  direction,  less than 001 greater than  direction,  less than 0-10 greater than  direction and  less than 00-1 greater than  direction from the center of the inner edge of the current constriction layer; and
(c) oxidizing the layer for forming the current constriction layer from lateral surfaces to form the current constriction layer having a substantially the same shape as the designed predetermined shape.
According to this manufacturing method, the shape of the inner edge of the current constriction layer is previously designed as a predetermined shape, and then based on the designed shape of the inner edge of the current constriction layer, the pillar-form semiconductor deposition is formed with its periphery of the predetermined shape. That is to say, the pillar-form semiconductor deposition is formed with its periphery reflecting the shape of the inner edge of the current constriction layer, and a current constriction layer can be formed having an inner edge substantially the same shape as the designed shape. In particular, by forming the inner edge of the current constriction layer to be a circle, a surface emitting semiconductor laser can be obtained which emits laser light with a beam shape closer to a true circle.
The method of manufacturing a surface emitting semiconductor laser has some features such as following examples (1) to (5).
(1) In the step (b), the arc-shaped portions and the parts of the designed inner edge of the current constriction layer may be formed in approximately concentric circles.
(2) In the step (b), the arc-shaped portions may be formed by splitting the designed inner edge of the current constriction layer in the  less than 01-1 greater than  direction,  less than 011 greater than  direction,  less than 0-11 greater than  direction and  less than 0-1-1 greater than  direction from the center of the inner edge, and by projecting the split parts of the inner edge of the current constriction layer respectively in the  less than 010 greater than  direction,  less than 001 greater than  direction,  less than 0-10 greater than  direction and  less than 00-1 greater than  direction.
(3) In the step (b), the distances from the center of the designed inner edge of the current constriction layer to the arc-shaped portions may be approximately equal.
(4) In the step (b), the arc-shaped portions may be formed to have the shape of the designed inner edge of the current constriction layer divided into fourth.
(5) In the step (b), the periphery of the pillar-form semiconductor deposition except for the arc-shaped portions may be formed outside line segments joining adjacent of the arc-shaped portions, in the plane including the designed inner edge of the current constriction layer.